The seat back of a motor vehicle seat with an integrated seat belt has a frame having two side parts and an upper base connecting the side parts and an upper belt point provided in proximity to the first upper corner. It further contains an L-shaped supporting body. The supporting body has a long leg connected to the frame in proximity to a first upper corner, of a length substantially equalling the length of the neighbouring side part and extending at an acute angle with respect to the neighbouring side part. A short leg is connected to the upper base, and a lower carrier piece connects a lower end of the long leg to the neighbouring side part so that the neighbouring side part, the long leg and the carrier piece form an acute-angled triangle.